1. Field of the Invention
This invention relates to thermoelectric devices and applications of the same as coolers/heaters or power converters. The thermoelectric devices utilize thin film and/or superlattice technologies to improve the materials properties and improve device performance.
2. Discussion of the Background
Application of solid state thermoelectric cooling is expected to improve the performance of electronics and sensors such as for example RF receiver front-ends, infrared (IR) imagers, ultra-sensitive magnetic signature sensors, and superconducting electronics. Bulk thermoelectric materials typically based on p-BixSb2−xTe3 and n-Bi2Te3−xSex alloys have figures-of-merit (ZT) or coefficients of performance (COP) which result in poor thermoelectric device performance.
The performance of thermoelectric devices depends on the figure-of-merit (ZT) of the material given byZT=(α2T/σKT)   (1)
where α, T, σ, KT are the Seebeck coefficient, absolute temperature, electrical condcutivity, and total thermal conductivity, respectively. Z, the material-coefficient, can be expressed in terms of lattice thermal conductivity (KL), electronic thermal conductivity (Ke) and carrier mobility (μ), for a given carrier density (ρ) and the corresponding α, yielding eqn. (2).
                    Z        =                                                            α                2                            ⁢              σ                                                      K                L                            +                              K                e                                              ≃                                    α              2                                                      (                                                      K                    L                                                        μ                    ⁢                                                                                  ⁢                    p                    ⁢                                                                                  ⁢                    q                                                  )                            +                                                L                  o                                ⁢                T                                                                        (        2        )            Here, L0 is the Lorenz number, approximately 1.5×10−8V2/K2 in non-degenerate semiconductors. State-of-the-art thermoelectric devices utilize alloys, typically p-BixSb2−xTe3−ySey (x˜0.5, y˜0.12) and n-Bi2(SeyTe1−y)3 (y˜0.05) for the 200K-400K temperature range. For certain alloys, KL can be reduced more strongly than μ leading to enhanced ZT.
A ZT of 0.75 at 300K in p-type BixSb2−xTe3 (x˜1) was reported forty years ago. See for example Wright, D. A., Nature vol. 181, pp. 834 (1958). Since then, there has been modest progress in the ZT of thermoelectric materials near 300K. The highest ZT in any bulk thermoelectric material at 300K appears to be ˜1.14 for p-type (Bi2Te3)0.25 (Sb2Te3)0.72 (Sb2Se3)0 03 alloy. See for example Ettenberg, M. H., Jesser, W. A., & Rosi, F. D., “A new n-type and improved p-type pseudo-ternary (Bi2Te3)(Sb2Te3)(Sb2Se3) alloy for Peltier cooling,” Proc. of 15th Inter. Conf. on Thermoelectrics, IEEE Catalog. No. 96TH8169, pp. 52-56 (1996).
Several approaches have been investigated to enhance ZT.
In bulk materials, cage-like structures simulating a phonon glass/electron crystal have been examined for reducing KL without deteriorating the electronic mobilities. See for example Slack, G. A. & Tsoukala, V. G., “Some properties of semiconducting IrSb3,” J. Appl. Phys. vol. 76, pp. 1665-1671 (1994).
A ZT greater than 1 has been reported in LaFe3CoSb12 at T>700K, attributed primarily to reduction of KL from La-filling. See for example Sales, B. C., Mandrus, D. & Williams, R. K., “Filled sketturudite antimonides: a new class of thermoelectric materials,” Science vol. 272, pp. 1325-1328 (1996). A ZT of approximately 1.35 was reported in CeFe35Co0.5Sb12 where, although a dramatic reduction in KL of the filled skuterrudite was observed near 300K, there was apparently less role of La-filling at higher temperatures where the enhanced ZT was observed. 900K. See for example Venkatasubramanian, R., “Cascade Cryogenic Thermoelectric Cooler,” U.S. Patent Application No. 60/190,924; the entire contents of which are incorporated herein by reference. A ZT significantly greater than 1 has not been demonstrated at ordinary temperatures (300K).
A one-to-one correlation between lower KL and enhanced ZT has not been established. More importantly, the concept of individually tailoring the phonon properties for thermal conductivity reduction without deteriorating electronic transport, thereby enhancing ZT, has not been established, prior to the present invention. Thin-film thermoelectric materials offer a tremendous scope for ZT enhancement, and three generic approaches have been disclosed.
One approach involves the use of quantum-confinement effects to obtain an enhanced density-of-states near Fermi-energy. See for example Hicks, L. D. & Dresselhaus, M. D. Effect of quantum-well structures on the thermoelectric figure of merit. Phys. Rev. B 47, pp. 12727-12731 (1993).
A second approach involves phonon-blocking/electron transmitting superlattices. See for example Venkatasubramanian, R., “Thin-film superlattice and quantum-well structures—a new approach to high-performance thermoelectric materials,” Naval Res. Rev. vol. 58, pp. 31-40 (1996). See also Venkatasubramanian, R. et. al, “Organometallic Epitaxy of Bi2Te3 and Related Materials and the Development of Planar, Monolithically-Interconnected, Superlattice-Structured, High-Efficiency Thermoelectric Elements”, Proc. Of 1st National Thermogenic Cooler Workshop (ed. Horn, S. B.) 196-231 (Center for Night Vision and Electro-Optics, Fort Belvoir, Va., 1992). See also Venkatasubramanian R. and Colpitts. T., Material Research Society Symposium Proceedings, Vol. 478, p.73, (1997). See also Venkatasubramanian R., Timmons, M. L., and Hutchby. J. A., Proc. Of 12th International Conf. On Thermoelectrics, Yokohama, ed. by K. Matsuura, 322, 1 (1993). These structures utilize acoustic-mismatch between the superlattice components to reduce KL while avoiding the conventional alloying for reducing KL, thus potentially eliminating alloy scattering of carriers. See for example Venkatasubramanian, R., Naval Res. Rev. Vol. 58, pp. 31-40 (1996), R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O'Quinn, 18th International Conference on Thermoelectrics, IEEE (1999) p. 100-103, and Venkatasubramanian, R. et al., “Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics,” Appl. Phys. Lett. vol. 75, pp. 1104-1106, (1999).
A third approach is based on thermionic effects in heterostructures. See for example Mahan, G. D. & Woods, L. M., “Multilayer thermionic refrigeration,” Phys. Rev. Lett. Vol. 80, pp. 4016-4019 (1998), and Shakouri, A. & Bowers, J. E., “Heterostructure integrated thermionic coolers,” Appl. Phys. Lett. vol. 71, pp. 1234-1236 (1997).
However, in all these approaches the degree of acoustic mismatch between the thermoelectric heterostructures has not been applied to thermally conducting structures without simultaneously deteriorating the electronic transport or with simultaneously enhancing the electronic transport or with simultaneously removing the electrical anisotropy and therefore significantly limiting the enhanced thermoelectric performance of the ensuing materials and devices disclosed therein.